This invention relates to voltage controlled crystal oscillators, and in particular, to crystal oscillators which can be tuned, temperature compensated, or synchronized.
For generating frequency reference signals in radio telephones and pagers, quartz crystal based oscillators predominate. Quartz crystal resonators offer several comparative advantages; they are inert, relatively power efficient, frequency stable and size scalable. However advantageous, crystal resonators present some practical problems. When quartz crystal is manufactured in an economical manner, its resonant frequencies cannot be predicted (or controlled) with an accuracy sufficient for many applications. Furthermore, the oscillating frequency of known quartz crystals is temperature dependantxe2x80x94the sensitivity varying according to crystal cut and crystal quality generally.
Accordingly, crystal oscillator circuits are both factory tuned to account for manufacturing variances and also equipped with features for temperature compensation. In the basic circuit design, an inverter and biasing resistor are each connected in parallel with the crystal resonator. The inverter and biasing resistor serve to start and then maintain the oscillation. An adjustable capacitance element such as a varactor is connected to the quartz crystal to allow frequency adjustment for factory tuning and temperature compensation. A voltage responsive temperature sensing element is scaled and operably connected to the adjustable capacitance element to provide temperature compensation of the oscillator frequency.
This frequency adjustment is conventionally called xe2x80x9cwarpingxe2x80x9d or xe2x80x9cpulling,xe2x80x9d labels which reflect the relative difficulty in changing the frequency of crystal-based oscillators. The stability of crystal circuits with greater adjustability (or gain) is generally more fragile because of increased susceptibility to noise. Accordingly, a compromise exists in oscillator design between increased frequency adjustability and stability.
Although such crystal-based oscillator circuits have received widespread commercial acceptance, efforts at improvement on this basic design continued. For example, U.S. Pat. No. 5,994,970 to Cole et al. describes a temperature compensation circuit employing a switched capacitor array in which the capacitor switch settings are continually adjusted in response to temperature changes. This approach of dynamically adjusting the settings of a switched capacitor array requires special controls to prevent related capacitor discharges from disrupting the oscillator reference signal. U.S. Pat. No. 4,827,226 to Connell et al. is directed to an oscillator circuit with a chip-integrated set of abrupt junction varactors for adjusting a capacitive load and the resulting oscillator frequency. Unfortunately, such integrated junction varactors occupy a relatively large area on a semiconductor chip. In summary, these and other conventional approaches suffer from one or more serious drawbacks including insufficient frequency gain, poor capacitive load selectivity, increased noise sensitivity, excessive manufacturing cost and chip integration incompatibility.
In the interest of allowing wireless communication providers to provide additional service, governments worldwide have allocated new higher RF frequencies for commercial use. To better exploit these newly allocated frequencies, standard setting organizations have adopted bandwith specifications with compressed transmit and receive bands as well as individual channels. These trends are pushing the limits of oscillator technology to provide sufficient frequency selectivity.
Coupled with the tighter frequency control requirements are the consumer market trends towards ever smaller wireless communication devices (e.g. handsets) and longer battery life. Combined, these trends place difficult constraints on the design of wireless components such as oscillators. Oscillator designers may not simply add more space-taking components or increase power dissipation in order to provide improved accuracy and stability.
Therefore, the need continues for improved oscillators which can offer frequency selectivity, size reduction and other performance improvements.
A controllable oscillator suitable for use in generating reference signals for signal frequency control in wireless communication devices is provided. The oscillator includes a crystal resonator having first and second electrodes, a gain stage having first and second terminals connected to the first and second electrodes for starting and maintaining the oscillation, first and second capacitor banks connected to the first and second terminals for providing a capacitive load to the resonator, and first and second dynamic switches interconnected between the terminals and the respective capacitor banks for cycling the capacitor banks in and out of connection.
The first bank of capacitors provides a capacitive load to the crystal resonator at the first terminal, while the second bank of capacitors provides a capacitive load to the crystal resonator at the second terminal. Each bank of capacitors includes at least two capacitors connected in parallel connected to a reference voltage source such as local ground in a parallel configuration. And, each bank of capacitors includes at least one static switch operably connected to one of the two capacitors for switching the capacitor in and out and thereby selectively increasing and decreasing the capacitive load presented by the bank of capacitors.
The first dynamic switch is interconnected between the first terminal and the first bank of capacitors and has a control input for receiving a DC control voltage. The second dynamic switch is interconnected between the second terminal and the second bank of capacitors and has a separate control input for receiving a DC control voltage. Each dynamic switch is responsive to a voltage difference between the respective control input and the respective (first or second) terminal such that a selected DC voltage at the control input can cause the dynamic switch to connect a bank of capacitors to the crystal resonator for a fraction of the period of oscillation.
In a preferred embodiment, the first dynamic switch is a transistor with a gate which serves as a control input and first and second source/drain regions. The first source/drain is connected to the first terminal and the second source/drain is connected to the first bank of capacitors. The second dynamic switch is preferably a transistor having a gate (as control input) and first and second source/drain regions connected between the second terminal and the second bank of capacitors. For this preferred configuration, the transistor switch acts in response to the source/drain-to-gate DC voltage differential.
Described in more general terms, oscillators according to this invention include a crystal resonator circuit, a static frequency corrector operably coupled to the crystal resonator circuit for applying a time-fixed frequency adjustment to the output frequency, and a dynamic frequency adjuster operably coupled between the resonator circuit and the static frequency corrector for limiting the period in which the static frequency corrector is connected to the crystal resonator.
There are other advantages and features of this invention which will be more readily apparent from the following detailed description of the preferred embodiment of the invention, the drawings, and the appended claims.